Tankage is an important feature of any facility handling large quantities of liquid such as a petroleum refinery which may process several hundred thousand barrels per day of petroleum to manufacture a full slate of petroleum products ranging from liquefied petroleum gas (LPG or "bottled gas") through gasolines, kerosines and light fuel oil to heavy bunker oil (No. 6), asphalts and heavy lubricants. To maintain continuity of operation there must be a reserve of crude petroleum held in tanks and there must be storage for intermediate (e.g. gasoline components) and finished products of all types. The varied nature of material stored imposes a number of varied storage problems.
The simplest type of tank is constituted by a cylindrical wall and a roof in the form of a cone, generally termed "cone roof" tank. Such tanks are suitable for storage of low volatility crude oil and products.
Even at low volatility of the stored liquid, evaporation losses are substantial when the liquid is stored in standard tanks with gastight steel roofs. The cycles of temperature change from night to day and with the season generates a condition known as "breathing" of the gas volume above the liquid surface. The gas volume contracts on cooling and air is drawn into the vessel. Conversely, a rise in temperature will expand the gas volume, forcing the vapor-saturated air out through the tank vents. In each cycle, the gas phase reaches equilibrium with the stored liquid. On exhalation, some of the light ends of the stored product are carried away.
Losses of product also result from filling and withdrawal of product. When liquid is dispensed from a storage tank, air enters through the vents. When again filled, the air is forced out, again carrying light vapors with it. These filling losses are usually less than breathing losses, but can be significant, particularly in tankage subjected to a high rate of turn-over by frequent dispensing and refilling.
A variety of conservation means are available for reduction in losses of product by discharge of air saturated with volatile components of the stored liquid. The vents of cone roof tanks can be provided with pressure-vacuum valves. Such conservation vent imposes slight pressure on contents of the tank to inhibit exhalation until the tank pressure exceeds the valve setting. As tank contents cool, tending to inhalation, the valve opens when its vacuum setting is exceeded. Some products are stored in vessels designed to withstand the vapor pressure of the contents. Such pressure storage facilities are often of spherical or spheroidal shape. Other tankage is designed to adapt to volume change of the vapor by change in volume of the space enclosed by any of several devices of which the common gas holder is exemplary.
A conservation tank of particular choice is a cylindrical tank having an upper closure which floats on the liquid contents, the so-called floating roof tank. This class of tankage includes the original pan-type tank roof which has been largely superseded by the pontoon and double-pontoon types. In the double-deck type, the pontoons cover the entire area of liquid surface. With no air-vapor space above the liquid, evaporation losses are limited to those caused by the small peripheral exposure at the seals. Such a tank eliminates 90 to 95 percent of the evaporation losses.
There is practically no filling loss with floating roof tanks. For that reason, these tanks are most advantageous for the storage of volatile products at locations where extensive product movement is anticipated.
The pontoon type of floating roof is quite stable. The double-deck type is even more stable and practically eliminates possibility of boiling from solar heat applied to the deck. In addition to control of losses due to evaporation and filling, the floating roof type of closure eliminates moisture due to condensation from inhaled moist air and thus reduces corrosion problems. The lack of vapor space above the oil surface greatly reduces fire risks.
Inherent in the floating roof design is a narrow annular space between the periphery of the roof and the wall of the tank. This space must be great enough to accomodate for protrusions on the inner surface of the tank wall such as rivet heads and butt straps, as well as minor deviations of the tank wall from truly round. It is usual practice to provide elements for sealing this space in the form of depending skirts biased to press against the wall, resilient pads and the like. Due to irregularities of the wall surface, wear on the seal elements and like causes, the seal is not complete and some evaporation does occur with resultant loss of product and some cases, such as motor gasoline, product quality is adversely affected.
Such evaporation as may take place from floating roof tanks involves the lighter hydrocarbons of the stored liquid. In motor gasoline, these fractions are responsible for easy starting of internal-combustion engines and have a favorable influence on the anit-knock properties of motor fuels. One authority reports that a volumetric loss of 1.0 percent from a typical gasoline results in a rise of 5.degree. F. in the 10 percent point of the distillation curve; a decrease of 0.5 psi in Reid vapor pressure, and a 1-point loss in the octane rating of the fuel.